The invention relates to a device for controlling delivery of sheets from a stack to a machine for processing them.
The operation of successively taking the top sheet from a stack of sheets to form a layer of sheets for insertion into a printing or cutting machine is well-known and numerous existing devices serve this purpose. In the case of a sheet-by-sheet supply of solid fiber board or corrugated board, flatness is a frequent problem, since the top surface of the stack may vary considerably in level. This is because the thickness of a cardboard sheet may vary in dependence on a number of external factors such as the ambient humidity or the storage conditions. This provides a difference in height between the front and rear of the stack and also between the center and edges thereof.
It has already been proposed to obviate this disadvantage by means of a device described in CH 651 807, comprising a mechanism for lifting the stack, a sheet-inserting means comprising a gripping element, means for detecting the top level of the stack connected to a computer acting on an electric motor of the stack-lifting mechanism and means for raising the stack in dependence on the top level. When the front level detector cannot see the stack of sheets, a command pulse for raising the stack is sent to the driving motor, which sends a pulse driving the stack-lifting mechanism. Depending on the thickness of the stack of sheets, the duration of the pulse is varied to obtain a variable rise step, so as to bring the top surface of the stack to the same level after the departure of each sheet from the stack.
A device of this kind considerably improves the accuracy of the top level of the stack, more particularly of the front vertical surface. Nonetheless, the disadvantage of raising the stack of sheets by pulses is that the least offset between successive pulses may result in a variation in the step or pitch of the resulting layer of sheets. If the step tolerance exceeds a certain amount, the machine processing the layer of sheets stops and has t be restarted, resulting in substantial loss of production.